1. Field of the Invention
The present invention relates generally to an optical information storage device, and more particularly to an optical information recording/reproducing device for recording/reproducing an optical signal on/from a recording medium.
2. Description of the Related Art
An optical disk has received attention as a memory medium that becomes a core in the recent rapid development of multimedia, and it is usually accommodated in a cartridge case to be provided as an optical disk cartridge for practical use. The optical disk cartridge is loaded into an optical disk drive to perform reading/writing of data (information) from/to the optical disk by means of an optical pickup (optical head).
A recent optical disk drive intended to realize size reduction is composed of a fixed optical assembly including a laser diode, a polarization beam splitter for reflecting and transmitting a laser beam, and a photodetector for receiving reflected light from an optical disk, and a movable optical assembly including a carriage and an optical head having an objective lens and a beam raising mirror mounted on the carriage. The carriage is movable in the radial direction of the optical disk along a pair of rails by means of a voice coil motor.
A write-power laser beam emitted from the laser diode of the fixed optical assembly is first collimated by a collimator lens, next transmitted by the polarization beam splitter, next reflected by the beam raising mirror of the optical head, and finally focused on the optical disk by the objective lens, thereby writing data onto the optical disk. On the other hand, data reading is performed by directing a read-power laser beam onto the optical disk. Reflected light from the optical disk is first collimated by the objective lens, next reflected by the polarization beam splitter, and finally detected by the photodetector, thereby converting the detected optical signal into an electrical signal.
A plurality of grooves are formed on a substrate of the optical disk in a concentric or spiral fashion to guide a laser beam to be directed onto the substrate. A flat portion defined between any adjacent ones of the grooves is called a land. In a general optical disk in the prior art, either the lands or the grooves are used as recording tracks on which information is recorded. In a magneto-optical disk drive as a kind of optical disk drive, a read-power laser beam is directed on a magneto-optical disk, and reflected light from the magneto-optical disk enters beam separating means such as a Wollaston prism. The incident light is separated into a P-polarized light component and an S-polarized light component by the beam separating means. The P-polarized light component and the S-polarized light component are differentially detected by a two-division photodetector to thereby reproduce a magneto-optical signal. Thus, in the magneto-optical disk drive, it is necessary to differentially detect the P-polarized light component and the S-polarized light component of the reflected light and optimally reproduce the magneto-optical signal.
The reflected light from the magneto-optical disk entering the Wollaston prism is to maintain linear polarization. However, a phase difference occurs between the P-polarized light component and the S-polarized light component of the reflected light in an optical system including the beam raising mirror and the polarization beam splitter. Further, on the magneto-optical disk whose substrate is formed of polycarbonate, a phase difference due to birefringence is enhanced to cause undulation of a DC component of the regenerative signal.
To reduce the undulation of the DC component of the regenerative signal, a conventional optical head is configured so that a phase difference in the optical head is controlled to be reduced by suitably selecting phase differences in individual optical components of the optical head or by suitably combining the directions of the phase differences in the individual optical components. Accordingly, it is difficult to reduce the cost of each optical component because of the selection and control of the phase differences in the individual optical components. Further, a total phase difference in the optical system as a whole depends on the accuracies of a plurality of optical components, so that a variation in phase difference largely differs between individual magneto-optical disk drives, causing a reduction in yield.
Japanese Patent Laid-open No. 1-229445 has proposed inserting a phase plate in an optical path of the reflected light to compensate for a phase difference between the P-polarized light component and the S-polarized light component of the reflected light. However, while the phase difference differs between individual magneto-optical disk drives, this publication includes no description on means for correcting this phase difference differing between individual magneto-optical disk drives. Japanese Patent Laid-open No. 8-297883 has proposed using a Soleil-Babinet phase plate as the means for correcting this phase difference differing between individual magneto-optical disk drives. However, the use of such a phase plate invites an increase in size and cost of the disk drive. Japanese Patent Laid-open No. 1-230222 has proposed locating a phase compensator in an optical pickup to compensate for a phase difference. This method allows simple phase compensation.
In this method, phase compensation is performed by rotating the phase compensator. However, the rotation of the phase compensator causes a change in orientation of its crystal axes (optic axes), resulting in a deviation in polarization direction of reflected light incident on a Wollaston prism located downstream of the phase compensator, from an optimum direction. The Wollaston prism is located so that its crystal axes (optic axes) form 45xc2x0 with respect to the polarization plane of the P-polarized light component or the S-polarized light component of the reflected light. If the polarization direction of the reflected light incident on the Wollaston prism is deviated by the rotation of the phase compensator, from the orientation of the Wollaston prism, i.e., 45xc2x0, there occurs an offset in DC component of the regenerative signal.
It is therefore an object of the present invention to provide an optical information storage device which can adjust the phase difference of the reflected light and the direction of the polarization plane in individual storage devices to reduce the undulation and offset of the DC component of a regenerative signal.
In accordance with an aspect of the present invention, there is provided an optical information storage device comprising an optical source; an optical head having an objective lens for focusing light from the optical source onto a recording surface of an optical recording medium; a first photodetector for detecting a regenerative signal from reflected light from the optical recording medium; a second photodetector for detecting a tracking error signal and a focusing error signal from the reflected light; a beam splitter for separating the reflected light into a first beam directed toward the first photodetector and a second beam directed toward the second photodetector; a phase compensating mechanism provided between the beam splitter and the first photodetector for compensating for a phase difference of the first beam; beam separating means provided between the phase compensating mechanism and the first photodetector for separating the first beam into two beams having different polarization planes; and polarization plane rotating means for rotating a polarization plane of the first beam incident on the beam separating means.
Preferably, the phase compensating mechanism comprises a phase plate and means for supporting the phase plate so that the phase plate is rotationally adjustable about an axis perpendicular to an optical path of the first beam. The phase plate has a thickness such that a phase difference larger than the sum of the maximum values of variations in phase difference in all optical components constituting an optical system through which the reflected light passes is given to the first beam when the angle of incidence of the first beam on the phase plate is 0xc2x0.
Preferably, the optical information storage device further comprises a condenser lens for focusing the first beam on the first photodetector, and the beam separating means comprises a Wollaston prism. The polarization plane rotating means comprises a rotatable lens holder for commonly accommodating the condenser lens and the Wollaston prism, and an optical base for mounting the lens holder so that the lens holder is rotationally adjustable about the optical axis thereof. In this case, the polarization plane rotating means comprises means for rotating the Wollaston prism fixedly mounted in the lens barrel.
Alternatively, the polarization plane rotating means comprises a half-wave plate; a cylindrical support for accommodating the half-wave plate; and means for holding the cylindrical support so that the cylindrical support is rotationally adjustable about the optical axis thereof. The phase compensating mechanism is adjusted so as to compensate for the phase difference of the first beam so that an undulation amount of a DC component of the regenerative signal is minimized, and the polarization plane rotating means is adjusted so as to rotate the polarization plane of the first beam so that an offset amount of the DC component of the regenerative signal is minimized.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.